docs: document OpenWrt cfg80211 backport builds

27af5ad added the CFG80211_VERSION make variable for OpenWrt builds that
use a newer mac80211 backport than the target kernel, but it is only
described in a Makefile comment. Document how to pass it from an OpenWrt
package Makefile, link the notes from the installation steps, and add
the build script and the kernel/backports combinations verified for
issue #94.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/openwrt-backports/build.sh

@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# Compile a source copy against a prepared kernel and staged wireless backport.
+# Leaves the build directory for inspection; does not install or load modules.
+set -euo pipefail
+
+if [ "$#" -lt 4 ]; then
+    echo "Usage: $0 KERNEL_BUILD_DIR STAGED_INCLUDE_DIR MAC80211_SYMVERS CFG80211_VERSION [make arguments...]" >&2
+    exit 2
+fi
+
+kernel_dir=$(realpath "$1")
+include_dir=$(realpath "$2")
+symvers=$(realpath "$3")
+wireless_version=$4
+shift 4
+
+for required in "$kernel_dir/Makefile" "$kernel_dir/Module.symvers" \
+    "$include_dir/mac80211/net/cfg80211.h" \
+    "$include_dir/mac80211-backport/backport/autoconf.h" "$symvers"; do
+    if [ ! -f "$required" ]; then
+        echo "Missing build input: $required" >&2
+        exit 2
+    fi
+done
+
+repo_dir=$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)
+build_dir=$(mktemp -d "${TMPDIR:-/tmp}/aic8800-backports.XXXXXX")
+cp -a "$repo_dir/drivers/aic8800/." "$build_dir/"
+echo "Build directory: $build_dir"
+
+make -C "$kernel_dir" M="$build_dir" \
+    NOSTDINC_FLAGS="-I$include_dir/mac80211-backport/uapi -I$include_dir/mac80211-backport -I$include_dir/mac80211/uapi -I$include_dir/mac80211 -include backport/backport.h" \
+    KBUILD_EXTRA_SYMBOLS="$symvers" \
+    CFG80211_VERSION="$wireless_version" \
+    -j"${JOBS:-2}" "$@" modules
+
+for module in aic_load_fw/aic_load_fw.ko aic8800_fdrv/aic8800_fdrv.ko \
+    aic_zlp_quirk/aic_zlp_quirk.ko; do
+    test -s "$build_dir/$module"
+done
+echo "All three modules built successfully in $build_dir"